Cooling electrical apparatus



Oct. 24, 1944. E. VENABLE 2,351,249

COOLING ELECTRICAL APPARATUS Filed Sept. 27, 1940 2 Sheets-Sheet 1 ZJ 21 j 15X. A Gram/Zar' u A; Crystal/ine Oct. 24, 1944. VENABLE 2,361,249

COOLING ELECTRICAL APARATUS Filed Sept. 2.7. 1940 2 Sheets-,Sheet 2 Patented Oct. 24, 1944 2,361,249 comme mormosn ArramrrusA Emerson Venable, Wilkinsburg, Pa., assignor toy Westinghouse Electric & `Manufaeturing Company, East Pittsburgh,

Pennsylvania Pa., a corporation of Application September 27, 1940, Serial No. 358,649

7 Claims. (Cl. F15-361) 'I'his invention relates rto electrical apparatus, ,Y

and, more particularly, to cooling electrical apparatus by means of solid materials.

In the manufacture of electrical apparatus, the design and construction are influenced by the fact that the electrical members are subject to heating when in use. Provision must be made ,to dissipate the accumulated heat for efcient operation of the apparatus. .The cooling of apparatus that has substantially no moving parts, such, for example, as transformers and reactors, introduces a problemN which is different from that found in rotating apparatus.

Heretofore the cooling of electrical apparatus, such as transformers, has been effected by means of fluids, particularly refined petroleum oils and These hydrocarbonv halogenated hydrocarbons. fluids are frequently preferred to other cooling agents, since they also contribute to the insulation of the electrical elements and thereby pere form a dual function.

These cooling fluids perform the function of4 cooling by convective heat transfer. The thermal conductivity through the fluids is so low that it may be considered negligible` for practical Ypurposes. Accordingly, the design pf the electrical elements in the cas/ing enclosing the transformer is influenced by the desire to obtain the best convective flow of the insulating liquid in order to keep the temperature within prescribed limits.

Under some operating conditions, petroleum oils deteriorate considerably and fail to function properly. Under arcing conditions, the oilwill tend to break down and form combustible gases. In addition, on exposure to air or oxygen, the oil will sludge and form a conducting precipitate. Occasionally, transformers filled with oil have caught fire or-even exploded, causing tremendous damage. Consequently, in certain localities where A such hazards would be dangerous, oil-filled transformer-s have been restrict/ed in their application or even prohibited.

It has been proposed to obviate the inflammable characteristics of refined petroleum oil by 'sub' stituting halogenated hydrocarbons therefor. The halogenated hydrocarbons are much more expensive than the rened petroleum oils employed in transformer constructions and are made use of only when the cost consideration is surmounted by the increased advantagesobtained by the lesser fire hazard.

In a few installations, air or gas-cooled transformers have been made use of in order to avoid these defects of liquid cooled apparatus. For example, n subways and in other localities where the presence of many persons is unavoidable, it has been common to employ transformers cooled byair due to the fact that the fire or explosion hazard is eliminated. However, air contains moisture and chemically reactive gases, such as carbon dioxide,`hydrogen sulphide and oxygen. These constituents, which are commonly present in air in relatively great quantities, are not desirable for electrical and chemical considerations. It is necessary to place a much heavier solid insulation upon the coils of air-cooled transformers in order that the coils will safely carry their load without failure. The insulation and the characteristics of air result in a greater thermal gradient to dissipate a given amount of heat from the electrical elements.

In all apparatus employing oils or air, the design emphasis must be placed upon providing suitable passageways and spaces within the electrical members in order to promote the best convective flow of cooling medium. The provision of numerous spaces and passageways does not coincide with the most efficient `electrical operation of the electrical apparatus or the most economical arrangement of the material for a given capacity. It is necessary to compromise the various requirements in order to obtai'n satisfactory operating results.

It is a purpose of this invention to replace fluid cooling mediums by dielectric solids which will function to dissipate the heat generated by the electrical elements by conduction alone. According to this invention, discrete crystalline particles or granules of solid inorganic salt or mineral material are used as a lling about the electrical elements to conduct or carry awaythe heat generated. The filling also acts as an insulation and results in a better electrical performance of the apparatus.

The object of this invention is to provide for cooling electrical apparatus.

A further object of ythis invention is to provide for insulating and cooling electrical apparatus by employing a dielectric mineral filler.

Other objects .and advantages will appear more fully from the following detailed description taken in conjunction with the accompanying drawings which are representative iof one embodiment of the invention, and in which:

Figure 1 is a vertical section of a transformer according to the invention;

Fig. 2 is an enlarged section of a portion of the apparatus of Fig. 1; and

Fig. 3 is a graph illustrating the improved heat transfer obtained through the use of the selected mineral filler.

The cooling of electrical apparatus by lmeans of liquids or gases depends almost entirely upon convective heat transfer. The thermal conduction of heat by liquids or gases is almost negligible in comparison with the convective heat transfer. For example, petroleum has an average thermal conductivity of about 0.00035 calorie per second per square centimeter per centimeter thickness per degree centigrade. By comparison, copper has about 3,000 times and fluorite has about 80 times this thermal conductivity.

For the purposes of this invention, it is proposed to use a dielectric composed of loose crystalline granules of minerals that have thermal conductivities of 50 times or more than that of oil. While it is a general rule that substances which have good thermal conductivity also have good electrical conductivity, this rule does not apply in the case of a few critical minerals which it is preferred to employ in this invention. Among these minerals are fiuorite (Calin). halite (NaCl), villiaumite (NaF), Potassium uoride (KF) and sylvite (KCl), which have thermal conductivities of 0.026, 0.0168, 0.025, 0.016 and 0.0166 calorie per square centimeter per degree centigrade per inch thickness, respectively.

In addition. these five minerals have an exceedingly high electrical resistivity up to the temperature of fusion. For example; at room temperature, the electrical resistivity of these mnerals is of the order of 101" ohms per cubic centimeter. At 100 C. thereslstivity is of`the order of 1013 ohms, and at500 C. the resistance is of the order of ohms.

The mineral salts fluorite, halite, villiaumite,

i Various other inorganic and mineral substances potassium nuoride, and sylvite are given as examples of materials presenting the required fun# damental thermal and electrical characteristics for a good filling for electrical equipment. Other materials having a thermal conductivity of 0.01 calorie or better in c. g. s. units and electrical resistance of the order above specified will be satisfactory.

It is a well-known fact in the transfer of heatV -that frequently the greatest resistance to heat flow is at the contact surfaces between two different elementsirl the path of the heat flow. Certain minerals have inherent high specific thermal conductivity, quartz, for example, but with a poor cleavage structure and the surface contact between adjacent particles is so poor that the overall thermal conductivity is much lower than that of a mass of other minerals with lower heat conductivity, but with better surface characteristics. It is believed lthat due to the poor cleavage of the quartz, which gives a conchoidal fracture. the contact between adjacent quartz particles is substantially a point-to-point contact with a consequent poor heat transfer.

The best overall heat transfer between a plurality of crystals of mineral will be had when the crystals are simple geometrical shapes with substantially all plane faces or with plane cleavagefractures. Cubic or tetragcnal crystals will pack together closely with plane face to plane face contact. Other simple geometrical solids will exhibitmr packing characteristics. A body of crystalsrso packed will exhibit the lowest thermal gradients. Buch a crystalline structure is present in fiuorite, halite, viiliaumite, potassium fluoride, and sylvite, all of which crystalline in the cub'cal or isometric system and have plane cleavage surfaces with regular angular relationwhich meet these` requirements of crystalline shape may be used as a cooling filling.

In employing ln. dielectric nller comprising such crystals, a further advantage obtains in/that a greater apparent density or packing coefficient is had with this type of crystal. A given volume was successively filled with crushed quartz'and crushed nuorite passing through the same mesh screen. The packing coemcicnt oi' the' i'iuorite was 89%, whereas that of quartz was only 80%. This means that thefiuorite nlled lallthe space more completely than dd the quartz, and consequently fewer voids were presenti" In addition, such av high packing coeiiicient necessarily results in a considerable amount of crystals having plane surfaces contacting plane surfaces on adjacent crystals. Therefore, uorite crystals should give bettercooling of electrical apparatus thana quartz lling.

Referring to Fig. 3 of the drawings, there is lllustrated the thermal performance of masses of three different materials placed within` a container and subjected to heatingfor a given period of time and cooling thereafter. The three different materials that were employed as a loose filling around the electrical member for the purpose of this test were fluorite. air and quarta The temperatures were taken at a position remote from the heating element to determine the A overall thermal conductivityof the filling material. It will be noted that the fiuorite indicated the maximum temperature at the remote point. In other words. its conductivity exceeded that of either quartz or air. It will be noted that the fluorite has a decided advantage over air as a thermal conductor as far as coolingefllciency is concerned. Quartz has g higher conductivity as a mineral, but, due to its. poor crystalline structure, was much inferior to either iluorite or aix as a heat conductor.

A transformer having electrical elements which are subject to heating when in use. when cooled by means of the dielectric mineral filling according to this invention, will have its heat dissipated most effectively if certain rencments in the design are made. It will be appreciated that heat must be transferred from the electrical elements, which will be covered with a certain amount of solid insulation, to the adjacent granular or crystalline mineral body. The optimum heat transfer will occur if the mineral crystals contact the electrical' elements at plane faces or fiat surfaces with as great aproportion as possible of the element being covered. This will insure that a low thema! gradient occurs at this point. Since the heat outflow areas are necessarily a minimum at the electrical elementI surfaces. it will be advantageous to increase the nat face contact and the areas in contact betwem the mineral .with the electrical elements in order to obtain lthe best overall heat conduction.

Finally, the transformer enclosure or casing will receive the heat after it has passed through the mineral body and will deliver it to the atmosphere. The transformer casing should be constructed so that the dat crystal faces will contact flat surfaces on the transformer casing for best thermal heat transfer. y All theseconsiderations aretaken advantage-of in the design of the apparatus shown on the drawings.

Referring to Fig. 1 of the drawings, the transformer Il comprises a bottom plate i2, side walls ascissa I4 havin8 fiat surfaces and' a cover I6. The transformer is hermetically sealed by means of welding i8. The cover I6 has an aperture 2i 'for the introduction of the loose granular mineral dielectric. The aperture 2l may be tightly sealed by means of a cover and appropriate sealing means. Attached to and extending through the cover i6 are two terminal bushings 22 and 24. Current passes by means of the terminal bush-v ings'to the conductors 26 connected with the electricalelements within the'transformer casing.

Operating electrical members within the casing comprise a core of magnetic material, preferably in laminated form, and a plurality of conductor pancake coils 32. Insulating bushings are placed about the legs of the transformer on which the coils 32 are wound. Thin insulation 34 is placed about each of the coils 32A to provide a certain amount of electrical insulation.

The coilsA and magnetic core, when placed within a transformer and connected up to the bushings 22 and 24 with the cover `i6 in place, are filled with the granular dielectric mineral filling selected from one or more of fiuorite, halite, sodium fluoride, potassium fluoride, and sylvite hereinbefore described. There are several methods of insuring a thorough filling of the transformer with the mineral to obtain the best packing. It is possible to vibrate the casing i4 with an electromagnetic vibrator to insure as cornplete filling as possible when the mineral granules are being poured in through opening 2l. The coils 32 may be energized with alternating current, and a certain amount of vibration will be generated to assist in a good placement ofthe minerai granules in and around the coil and core.

In some instances, it may be more satisfactory to place selected mineral particles between the coils before assembling the electrical elements together. Referring to Fig.. 2 of thedrawings, it will be noted that they coils 32 are separated by closely packed layers of salt or mineral 42. These may be fitted in by hand before the apparatus is assembled. A convenient method of doing this will be to take an insulated pancake coil 32, apply it to the sleeve 36 in a vertical position, and

ribbed casings may also be used for this purpose. i It will be appreciated that the best filling will be had when the loose salt filling 46 consists of account.

. sions.

crystals all having substantially the same physical dimensions. Controlled crystallization of a saturated salt solution will yield a high percentage of crystals having the same physical /dimensions. Close screening of the crystals, however obtained, is another possible way to secure uniformly sized particles.

It is not necessary to employ the natural crystal, but blocks or cubes of any desired physical size to suit the requirements of manufacture may be sawed or cut from large salt masses. In addition, the salt or mineral may be fused and cast into cubes or shapes of the selected dimen- The fused salt should recrystallize on cooling forgood thermal conductivity. The natural crystal will undoubtedly be the least expensive and, where obtainable in the proper size and uniformity of particles, will be preferred on this It will be appreciated thai'l finer crystals, due, to their relatively large number for a given vol-- ume, may retard heat flow. It would be more satisfactory to employ crystals or grains of large physical size, since the larger particles will have fewer interfaces for the heat to pass through.

In the largest type of transformer, such as used in major electrical systems, the transformer may be filled by laying large size blocks of salt by hand in uniform rows and building up the transformer into a solid and substantially voidless filling. The optimum thermal conductivity will be obtained by a transformer in which the dielectric filling is laid in such a regular manner.

It is preferred to enclose the entire transformer within a casing that is substantially hermetically sealed with respect to the outside air. The entry of moisture into the filling may result in 'corrosion and deterioration of the magnetic core and other elements, and this it is desired to avoid.

To insure that all the moisture is removed when to apply one or more layers of salt crystals of 1 unifonn shape. A dense and homogeneous salt filling will be effected. A second coil 32 may be slipped over the end of the insulating tube 36 and rested on thejlayers of crystals. Additional layers of salt crystals may be placed upon the second coil 32. Tape may be placed around the periphery of the salt layers to prevent their displacement as the operation is continued. When a complete stack of coils suflicient for one leg of the transformer has been built up in this manner, the leg of the core of the magnetic material may be inserted into the insulating sleeve 36. This may be readily accomplished due to the fact that the legs ofthe core of magnetic material are generally cut out in L-shape punchings and assembled one by one into cores of desired thickness. When both legs of the core of magnetic I the transformeris filled, the transformer may be heated to a high temperature in order that all of the moisture present will be driven off. A moisturevabsorbing material. such as calcium chloride. may be placed within the transformer to insure an extremely dry atmosphere for this purpose. Once the thoroughly dried transforme'` is sealed, however, the contents will be protected from chemical change.

, The minerals fiuorite, halite, villiau/mite: pO-g tassium fluoride, and sylvite have good electrical properties up te very high temperatures. Ther resistance is satisfactory even at a dull red heat. A transformer'constructed with the mineral or salt filling will be able to withstand tremendous overloads for long continued periods of time without suffering any permanent deterioration. The salt filling is a high grade insulatorand will withstand surges which would be'serious or even fatal in oil-filled transformers.v If fiashover or arcingoccurs, the mineral does not break down permanently. but at most may fuse. However. once the electrical stress is removed. the filling resolidifles at this place and electrically is as For best results, the walls I4 of the transformer casing should be made so as to presentv flat. surfaces to the salt or'mineral filling. A

square casing is one possible type of casing for this purpose. Hexagonal and corrugated fiat good as it originally was. This self-healing characteristic constitutes` a considerable improvement on hydrocarbon material which breaks down permanently and leaves a conducting carbonaceous residue. I

lThe preceding descriptionvrelates to the preferred embodiment of the invention. Changes direct contact with the electrical member colo operative to transfer accumulated heat to the casing to control the temperature, the dielectric comprising loose granular crystalline .parallelepiped shaped solids having a thermal conducrounding and contacting the electrical member and contacting the casing, the heat conducting dielectric filling composed of loose, crushed, granul'ar crystalline solids of substantially the same 4l5 size, the solids having a thermal conductivity of greater than 0.01 calorie per second per degree centigrade per square centimeter of surface for a thickness of one centimeter and having predominantly plane surfaces and plane cleavage fracture, and of simple geometrical shapes to provide for a high packing coefliclent, and a high proportion ofthe solids having plane surface to plane surface contact to effect good thermal conductivity through the dielectric filling to thereby tivity ,f 0 91 Calorie or higher per cgntimer per l5 effect transfer of the heat generated by the elecdegree centigrade per square centimeter per second, the granular solids having predominantly fiat surfaces and sufficient surface contact with one another, with the electrical member and with `trical member to the casing with a low thermal gradient.

5. Electrical apparatus comprising, in combination, a solid electrical conductor member subthe casing to effect a predetermined transfer ject t0 heating when in Operation 3 Casing for and dissipation of heat to the casing, the granular solids applied adjacent the electrical member as a substantially voidless body to effect efficient overall thermal conductivity.

the electrical conductor member, a good heat dissipating dielectric filling surrounding the 'elec trical member, the dielectric nlling composed of at least one salt selected from Athe group con- 2. In a transformer, in combination, electrical Sistmg of calcium uoride sodium chloride' 5' conductors and a core subject to heating when in use, a casing enclosing and protecting the electrical conductors and core, and a dielectric in the casing appliedf directly to ,the electrical conductors and the core cooperative to transfer heat to the casing to control the temperature, the dielectric comprising loose, granular crystalline parallelepiped shaped solids having a thermal conductivity of 0.01'calor'ie or higher per centimeter per degree centigrade per square cen 35 timeter per second, the crystalline solids havingI substantially all plane faces, the granular solids having sufilcient surface contact with one another along the plane faces, with the electrical conductors and the core, and with the casing to 4u eilect a predetermined transfer and dissipation of heat to the casing.

3. Electrical apparatus comprising, in combination, an electrical transformer core and windings subject to heating when in operation,v a casing having predominantly fiat walls enclosing and protecting the electrical transformer core and windings and anI electrical insulating and heat dissipating dielectric i'illingin the sealed casing, the filling surrounding and contacting the electrical transformer core and windings and contacting the flat walls of the casing, the heat conducting dielectric nlling composed of loose, granular crystalline solids having a thermal conductivity of 0.01 calorie or higher per centimeter per degree centigrade per square centimeter per second having predominantly plane surfaces and of simple geometrical shape t9 provide for a high packing coeilicient, and a high proportion of the crystalline solids having plane surface to plane surface contact to effect fgood thermal conductivity through the dielectric filling to thereby effect transfer of the heat generated by the electrical transformer core and windings to the walls of the casing with a low thermal gradient.

4. Electrical apparatus comprising, in combination, a solid electrical conductor member subelectric fllling the sealed casing, the filling sur.;A

coefficient and a high proportion of the solid bodies having plane face to plane face contact to provide for a good thermal conductivity to effect removal of the heat from the electrical member with a low thermal gradient.

6. Electrical apparatus comprising," in combination, a solid electrical conductor member subject to heating when in use, a sealed casing for enclosing and protecting the electrical member, and an electrical insulating and heat conducting dielectric filling in and contacting the walls of the casing surrounding the electrical member, thefllling composed of at least one salt selected from the group of calcium uoride, sodium chloride, sodium fluoride, potassium fluoride, and

s'potassium chloride, the salt filling composed of a plurality of bodies of simple, solid geometrical shape having plane faces and a plane cleavage g fracture to provide for a good packing coemcient and a high proportion of the solid bodies having plane face to plane face contact to provide for a good thermal conductivity to effect removal and dissipation of the heat developed in the electrical member to the casing with a low thermal gradient.`

7. A loose, granular dielectric nlling for solid g5 for a good packing coefilcient in the sealed casing whereby heat is removed from the electrical members and transferred to the casing with low thermal gradients, the filling having good electrical insulating properties at temperatures below the fusing temperature of the salts.

EMERSON VENABLE. 

